fix(rails): don't load ActionController::Live before initialization

sentry-rails/lib/sentry/rails/railtie.rb

@@ -6,6 +6,11 @@
 
 module Sentry
   class Railtie < ::Rails::Railtie
+    # Referencing ActionController::Live loads ActionDispatch::Response, and Rails
+    # warns when that happens before the application has initialized. Rails 8.1.4
+    # added the :action_controller_live load hook so we can wait for it instead.
+    SUPPORTS_ACTION_CONTROLLER_LIVE_LOAD_HOOK = Gem::Version.new(::Rails.version) >= Gem::Version.new("8.1.4")
+
     # middlewares can't be injected after initialize
     initializer "sentry.use_rack_middleware" do |app|
       # placed after all the file-sending middlewares so we can avoid unnecessary transactions
@@ -99,7 +104,16 @@ def extend_controller_methods
       ActiveSupport.on_load :action_controller do
         include Sentry::Rails::ControllerMethods
         include Sentry::Rails::ControllerTransaction
-        ActionController::Live.send(:prepend, Sentry::Rails::Overrides::StreamingReporter)
+      end
+
+      if SUPPORTS_ACTION_CONTROLLER_LIVE_LOAD_HOOK
+        ActiveSupport.on_load :action_controller_live do
+          prepend Sentry::Rails::Overrides::StreamingReporter
+        end
+      else
+        ActiveSupport.on_load :action_controller do
+          ActionController::Live.send(:prepend, Sentry::Rails::Overrides::StreamingReporter)
+        end
       end
     end
 

sentry-rails/spec/isolated/streaming_reporter_spec.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+begin
+  require "simplecov"
+  SimpleCov.command_name "StreamingReporter"
+rescue LoadError
+end
+
+require "shellwords"
+require "sentry/rails/overrides/streaming_reporter"
+
+RSpec.describe Sentry::Rails::Overrides::StreamingReporter do
+  it "is prepended to ActionController::Live" do
+    make_basic_app
+
+    expect(ActionController::Live.ancestors).to include(described_class)
+  end
+
+  # rspec-rails loads ActionController::Live before the suite starts, so boot a
+  # bare app in a child process to see what initialization alone loads.
+  it "doesn't load ActionController::Live while the app initializes" do
+    skip "Rails #{::Rails.version} has no :action_controller_live load hook" unless Sentry::Railtie::SUPPORTS_ACTION_CONTROLLER_LIVE_LOAD_HOOK
+
+    script = <<~RUBY
+      require "action_controller/railtie"
+      require "sentry-rails"
+
+      class LiveLoadApp < Rails::Application
+        config.eager_load = false
+        config.secret_key_base = "secret"
+      end
+
+      LiveLoadApp.initializer :sentry do
+        Sentry.init do |config|
+          config.dsn = "http://12345:67890@sentry.localdomain:3000/sentry/42"
+          config.sdk_logger = Logger.new(nil)
+          config.background_worker_threads = 0
+        end
+      end
+
+      LiveLoadApp.initialize!
+      # loading controllers must not drag ActionController::Live in with them
+      ActionController::Base
+
+      print ActionController.autoload?(:Live) ? "lazy" : "loaded"
+    RUBY
+
+    expect(`#{RbConfig.ruby} -e #{Shellwords.escape(script)}`.lines.last).to eq("lazy")
+  end
+end